Show Only Tasks Per Application in Task Report

There is no OOTB feature to get tasks per application. Context can be process models and we can use that to generate task report.

1) Create a Task report with context process model
2) Create a constant of type array of Process models and provide all the process models of application in that
3) In query analytics use this constant as context to report.

Another way of doing this (which just requires some advance planning) would be to pass around a text PV containing the name of the app a certain process model belongs to, which can then be used to filter the task report. Some process models could even be made cross-app by passing in a different value when called by different parent models.

+1 to Mike's design pattern. I would add that a number might make more sense (instead of text) depending on the volume of tasks you have, because string comparisons are expensive and could slow your report down if there are many tasks in your report.